Final answer:
Progesterone is the hormone that most directly influences the secretory phase of the menstrual cycle by preparing the endometrial lining for potential implantation.
Step-by-step explanation:
The hormone that most directly influences the secretory phase of the menstrual cycle is progesterone. During the secretory phase, progesterone from the corpus luteum begins to prepare the endometrium for potential implantation of a fertilized egg by enriching it with glycogen and other nutrients. This phase also involves the development of spiral arteries that provide blood to the engorged endometrial lining. Progesterone is essential for maintaining this enriched environment, suitable for an implanting embryo.
Although estrogen, FSH (Follicle-stimulating hormone), and LH (Luteinizing hormone) also play roles in the overall menstrual and ovarian cycles, it is progesterone that is pivotal in the secretory phase. Following ovulation, the corpus luteum, which is formed from the remains of the follicle, secretes high levels of progesterone. This shift from estrogen to progesterone dominance is what stimulates the uterus to enter the secretory phase.